The following U.S. patents are believed to be representative of the prior art:
U.S. Pat. Nos. 4,750,197; 5,056,837; 5,097,253; 5,127,687; 5,169,188; 5,189,396; 5,406,263; 5,421,177; 5,587,702; 5,656,996 and 6,069,563.